B and T lymphocytes for an interacting system of recognition and effectors cells which protects the animal from infection by a wide variety of microorganisms. Their recognition systems depend on clonotypic cell-surface receptors, immunoglobulin (Ig) on B cells and T cell receptor (TCR) on T cells, of a near-limitless range of specificities. This enormous diversity of receptor specificity is possible because the genes encoding IG and TCR subunits are assembled during lymphocyte development by a novel and highly regulated series of gene-segment rearrangements. The products of these rearranged genes themselves are involved in the regulation of lymphocyte development. This proposal outlines experiments aimed at understanding the role of Ig heavy-chain mu protein in regulating the lymphoid-specific recombinase activity. We propose to use bone marrow from recombinase-deficient Ig gene transgenic mice and fetal liver B cell progenitors isolated on successive days of mouse gestation to help separate the different stage of B cell development from one another. We previously devised techniques which allow us to detect and quantify Ig gene rearrangements, rearrangement reaction intermediates, and relevant gene transcripts using very small numbers of cells. We propose to use these new techniques and purified progenitor B cells to 1) determine the effect of Ig mu and kappa chains and complete IgM on the targeting of he recombinase; 2) elucidate the mechanisms and identify ligands involved in the regulation Ig kappa gene rearrangement by Ig mu protein; 3) test the involvement of the transcription factor NF-kappaB in the activation of Ig kappa gene transcription and rearrangement; and 4) determine the role of surface IgM in eh inactivation of gene rearrangement. Aberrant lymphocyte development can result in disease. For example, several different hematopoietic malignancies involve errors in Ig/TCR gene rearrangement which juxtapose a prot-oncogene with an active Ig or TCR gene. We envision our experiments as providing the ground work for eventual study of the developmental abnormalities of the immune system which lead to immunodeficiency, autoimmunity, and lymphoid malignancy.